Session Initiation Protocol (SIP) is an open signaling protocol for establishing many kinds of real-time communication sessions. Examples of the types of communication sessions that may be established using SIP include voice, video, and/or instant messaging. These communication sessions may be carried out on any type of communication device such as a personal computer, laptop computer, Personal Digital One key feature of SIP is its ability to use an end-user's Address of Record (AOR) as a single unifying public address for all communications. Thus, in a world of SIP-enhanced communications, a user's AOR becomes their single address that links the user to all of the communication devices associated with the user. Using this AOR, a caller can reach any one of the user's communication devices, also referred to as User Agents (UAs) without having to know each of the unique device addresses or phone numbers.
One of several steps that are performed during the establishment of a SIP session is contact resolution which determines which devices registered to a given user will alert on any given call. In traditional systems, contact resolution policies (decisions made regarding what device to ring, how to ring it, etc.) for a user were built into the PBX and operated only for provisioned telephone sets. Thus, a user's choices for contact resolution policies were PBX release and telephone device specific. This is not very desirable in SIP since the underlying IP network becomes a general purpose attachment mechanism for any number of SIP communications devices registered by the user. It is the SIP user who decides which of his or her devices are registered, attached, enabled, and to what purpose each of these devices will be used. In SIP, contact resolution is conventionally done in the location server, but like a PBX, it is often undesirable to re-deploy the location server to be able to offer new contact resolution behavior. It is therefore desirable for a user to have access to contact resolution policies that can be easily provided on their behalf by systems/network administrators without requiring a modification of the specific PBX or communications feature servers or location servers of the network in general.